Explainer: G.SHDSL: It's like ADSL, only better

G.SHDSL is a new international standard for single-pair, high-speed DSL, as defined in the ITU-T Standard G.991.2. Unlike asymmetric DSL, which was designed for residential applications in which more bandwidth is delivered downstream (to the house) than is available upstream (to the Internet), G.SHDSL is symmetrical - offering 2.3M bit/sec in both directions.

This makes G.SHDSL better-suited for business applications, which require higher-speed bandwidth in both directions.

G.SHDSL combines the positive aspects of existing copper-based, high-speed communications with the benefits of increased data rates, longer reach and less noise.

Today's North American private line, frame relay and Internet services for business applications typically are served by T-1 - 1.544M bit/sec access lines. T-1 technology moved from Alternate Mark Inversion/Bipolar 8 Zero Substitution (AMI/B8ZS) coding to high-bit-rate DSL (HDSL) in the early 1990s. T-1 AMI/B8ZS was a two-pair (four-wire) technology with limited reach, requiring a signal regenerator (repeater) 3,000 feet from the central office and another every 6,000 feet.

T-1 repeaters are expensive to purchase, install and maintain, but are required to deliver T-1 service. HDSL applied the new ISDN-based modulation scheme - 2 Binary 1 Quaternary - to T-1 communications, which resulted in transmission up to approximately 9,000 feet without the need for repeaters. North American telephone companies quickly migrated to HDSL to save the cost of one or two repeaters.

In the rest of the world, business applications are typically served by E-1, at 2.048M bit/sec. Europe, where business customers usually are within reach of the central office, has not migrated as quickly to HDSL transport.

But Europe, along with the rest of the world, did want to take advantage of the advances being made in the DSL world and, through the International Telecommunications Union, sanctioned the specification of G.SHDSL to provide increased bandwidth with reduced noise.

Today, U.S. business DSL lines are predominantly asymmetric DSL (ADSL) - the residential technology with deliverable data rates that cap out at 384K bit/sec for symmetrical service. North American telephone companies are evaluating G.SHDSL technology for Internet services targeted at small to midsize companies, offering data rates of 786K, 1.544M and 2.3M bit/sec. These Internet services will offer a reduced service-level agreement compared with T-1 or E-1 services, at a lower monthly cost.

Four factors are driving the interest in G.SHDSL:

Standardization: The industry needs a higher-speed digital transport service for business applications. HDSL was never adopted as an international standard. Symmetric DSL - introduced as the DSL service for businesses in the late 1990s - never became a standard and interfered with the residential ADSL service because it was spectrally incompatible (very noisy). G.SHDSL is positioned for deployment in Internet and T-1/E-1 infrastructure applications because of its international standardization.

Improved data rate: G.SHDSL offers a two-wire standard operating at 2.3M bit/sec and four-wire standard operating at 4.6M bit/sec. HDSL, when initially introduced, provided 1.544M bit/sec with four wires. G.SHDSL offers roughly three times that and, when compared with the newer HDSL2 and HDSL4 services (1.544M bit/sec over two wires or four wires), still provides significantly more bandwidth.

Improved reach: G.SHDSL generally provides 20 percent to 30 percent increase in reach over HDSL at the same deliverable data rates. Additionally, when G.SHDSL multilink technologies are used, such as four-wire, Inverse Multiplexing for ATM and permanent-virtual-circuit bonding, G.SHDSL's reach is more than double HDSL's.

Spectral compatibility: G.SHDSL is spectrally compatible with ADSL, causing little noise or crosstalk between cables. Therefore, G.SHDSL services can be mixed with ADSL in the same cables without much - if any - interference.

G.SHDSL quickly has caught on in European markets, and the major North American local exchange carriers will roll it out soon.

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